1. Field of the Invention
The present invention relates to microelectronics structures and a pattern reversal method of forming dark field features using bright field lithography.
2. Description of Related Art
As the semiconductor industry continues to advance, development of a photolithography method to shrink features, such as trenches and holes is urgently needed. Some ancillary methods have been developed to help push the progress of photolithography. For bright field lithography, in which only small portions of the photoresist, such as lines, are protected from the exposure and remain after development, overexposing can be used to shrink the size of the features. Moreover, one can also trim features by dry etching to reduce their width. Those technologies are effective down to features smaller than 32 nm. However, no comparable method exists to shrink trenches, spaces, or holes (e.g., contact, via). In addition, these methods still face the issue of feature collapse as features get smaller and smaller. One way to address the problem of feature collapse is to use thinner and thinner photoresist layers. However, the very thin photoresist layer is not sufficiently etch resistant, thereby causing problems during pattern transfer.
In current integrated circuit (IC) production, trenches and contacts holes are fabricated using dark field lithography in which a large portion of the photoresist is protected from exposure, while only small portions of the photoresist are exposed and removed after development. Dark field lithography and its ancillary technologies, such as resist reflow or Resolution Enhancement Lithography Assisted by Chemical Shrink (RELACS), have serious technical difficulties to overcome in order to retain good critical dimension (CD) control when features are 45 nm or smaller. An effective method for fabricating dark field features, such as trenches and holes, is urgently needed to achieve progress in light field lithography.
Reversal lithography, an approach to forming dark field features based on bright field lithography technology, has been proposed as one alternate strategy. For this approach, a template is first patterned on the substrate for example, using a photoresist and bright field lithography. Next, a material that etches much slower in oxygen plasma than the photoresist is coated on the template, filling the holes and trenches and overcoating the tops of the features (e.g., lines) formed in the photoresist. Because the tops of features are always overcoated in this process, an etch-back process is then necessary to open the cap of the features. After the tops of the features are clear, oxygen etching is used to remove the photoresist without significantly etching the reversal material, which remains on the substrate. As a result, the original photoresist features created by the bright field lithography are reversed into dark field features, such as trenches. However, this method has not achieved wide acceptance due to its process complexity and the required etch-back steps, which greatly increase cost. To minimize the amount of etch-back required, a coating that is as thin as possible has been proposed. However, this raises other issues, such as coating uniformity. That is, when a thin coating is applied on a surface with different features, the size and density of the features have a strong impact on coating thickness. The uneven thickness causes defects to be generated during subsequent etch processes, which is still a serious obstacle for this technology to be accepted by the industry. Thus, there remains a need for a method of producing dark field features, such as trenches and holes at features sizes below 45 nm.